On internal combustion engines configured to change an engine compression ratio, the engine compression ratio is controlled to a relatively high compression ratio during low load operation and also controlled to a relatively low compression ratio during high load operation. Therefore, when the operating condition changes from a low-load operating condition to a high-load operating condition, for example, during an accelerating condition, the engine compression ratio is reduced in accordance with an increase in load. Assuming that an actual compression ratio becomes higher than a target compression ratio due to a response delay of the engine compression ratio when reducing the engine compression ratio in accordance with a change in the operating condition in the foregoing manner, knocking may possibly occur.
For instance, Patent document 1 discloses an internal combustion engine control device configured to calculate a second target compression ratio by performing phase-lead processing, in which a response delay of the compression ratio caused by a variable compression ratio mechanism is taken into account, with respect to a first target compression ratio calculated from a current engine load (intake air quantity), and to control the compression ratio closer to the second target compression ratio.
In this Patent document 1, in the case that the responsiveness of a compression ratio varied by a variable compression ratio mechanism is superior to the responsiveness of an intake air quantity, the superior responsiveness of the compression ratio permits the second target compression ratio to be calculated. Therefore, it is possible to bring the actual compression ratio sufficiently closer to the first target compression ratio corresponding to an original compression ratio.
However, conversely in the case that the responsiveness of an intake air quantity is superior to the responsiveness of a compression ratio varied by the variable compression ratio mechanism, there is a possibility that the second target compression ratio cannot follow the first target compression ratio.